1. Field of the Invention
The present invention relates to the fields of nuclear magnetic resonance spectroscopy and magnetic resonance (MR) imaging.
2. Discussion of Prior Art
Presently, the acquisition of Magnetic Resonance (MR) images is a useful procedure in the diagnosis of many diseases in humans. In these procedures a subject is placed in a magnet causing resonating nuclei of the subject, or "nuclear spins", to generate longitudinal spin magnetization. In a common procedure, this magnetization is detected by the application of a radio-frequency (RF) pulse capable of nutating the longitudinal spin magnetization by a selected amount to create transverse spin magnetization. Maximum transverse spin magnetization is generated by the application of a 90.degree. nutation. Unlike longitudinal magnetization, transverse spin magnetization is capable of inducing a signal in a receiver coil placed near the sample.
The signal induced in a receiver coil carries significant information about the local environment of signal generating nuclei. If the signal is acquired in a homogeneous magnetic field, the spectral components of the signal can be resolved to provide a nuclear magnetic spectrum in which different peaks arise from populations of nuclei in different molecules (or parts of a molecule).
If the spatial distribution of transverse spin magnetization is to be measured, as in MR Imaging (MRI), the transverse spin magnetization can be phase shifted with the application of magnetic field gradient pulses of selected intensities and durations. These gradient-induced phase shifts encode the position of spin magnetization within the magnet. Two or three-dimensional images of the distribution of spin magnetization can be generated by repeating the sequence of RF and magnetic field gradient pulses and acquiring the MR signal responsive to a collection of magnetic field gradient intensities.
In typical imaging pulse sequences image dimensions are obtained responsive to either a frequency-encoding (sometimes called readout) gradient pulse or a phase-encoding gradient pulse. It is possible to apply two phase-encoding gradient pulses simultaneously in different selected directions. By necessity, however, a frequency-encoding pulse cannot be applied simultaneously with a phase-encoding pulse. Consequently, position measurement in the frequency-encoding and phase-encoding dimensions cannot occur at the same instant in time. While this is not a problem when the imaged objects are stationary, it creates a registration artifact for those nuclear spins which are moving with velocity components in both the frequency and phase-encoding dimensions. This effect can create significant image distortions in the region of flowing blood within a subject.
A method currently in use which minimizes artifacts due to motion is the incorporation of flow-compensation gradient waveforms into the imaging pulse sequence. These waveforms are applied between the excitation RF pulse and data detection. The shape of the waveform is chosen to cause the first moment of the entire gradient waveform with respect to some time, t, to be zero. Flow-compensation gradient waveforms are used to minimize or eliminate phase shifts arising from motion during the slice-select, phase-encoding and frequency-encoding portions of an imaging pulse sequence. Correction of misregistration artifacts can be accomplished by incorporating flow-compensated gradient waveforms into both the frequency- and phase-encoding gradient waveforms. Incorporation of these compensation waveforms, however, increases the minimum echo time, TE, and cannot be used with bi-polar phase-encoding gradient waveforms such as those used to acquire Fourier velocity encoded images. A complete as description of flow-compensation waveforms can be found in "Elimination of Oblique Flow Artifacts in Magnetic Resonance Imaging" by L. R. Frank, A. P. Crawley, R. B. Buxton, Magn. Reson. in Med. 25: 299-307 (1992) ("Frank publication").